Unfortunately basing market price around offers made by potential buyers can sometimes be a little misleading. Extreme desire for an item, or lack of knowledge can sometimes lead to inflated offers. Of course, the same is also true in reverse (potential buyers might not be fully aware of the market price, and so offer something substantially less than that)!
I have three original books in my personal collection (the earliest example, through to the version shown here). Early Booklets (those with linen-effect tags and copper wire ties) are now demanding a price somewhere between €300 and €400. However, the example that Pete's shown here is actually the later variant (1942 revision). As such, I'd estimate its price to be more in the region of £100 - £150, assuming that it's complete and intact. It's difficult to ascertain that from the pictures here (for example is it complete with all duplicate tags, carbon paper, string ties, etc.), but it does look complete from the illustrations Pete has shared with us.